1. Field of the Invention
The present invention relates to a lead material for an electronic part, a lead and a semiconductor device using the same. More particularly, the present invention relates to a lead material for an electronic part wherein the surface of a conductive substrate is coated with a plated layer made of a Sn group material which does not contain Pb. The lead material does not give an adverse influence to the environment because it does not contain no Pb, and also has excellent solderability (or solder wettability) and provides a strong junction with a solder, and does not cause nonuniform thickness of the plated layer even in reflow processing.
2. Description of the Related Art
A lead material having a conductive substrate, such as a Cu substance or a Cu alloy, of which surface is coated with a plated layer made of a Sn substance or a Sn alloy represented by a solder, is a high-performance conductor which not only shows an excellent conductive property and mechanical strength based on the characteristics of the Cu substance or the Cu alloy, but also shows corrosion resistance and good solderability based on the characteristics of the Sn substance or the Sn alloy. The lead material is widely used in such field as electronic/electric equipment field as various types of terminals, connectors and leads or a power cable field.
When mounting semiconductor chips on a circuit board, the outer lead part of a semiconductor chip is subjected to solder by hot dipping or electroplating, thereby improving the solderability of the outer lead part.
However, in the above-stated lead material, if the plated layer coating the conductive substrate is made of a Sn substance, the following problems can occur.
First, Sn whiskers (acicular single crystals) grow up on the Sn plated layer thus formed, which cause a short-circuit accident. This problem can be solved by applying reflow-processing to the Sn plated layer.
Nonetheless, the melting point of the Sn substance is relatively high, i.e., 232.degree. C., and the Sn plated layer is easily oxidized by the environment of the heat applied during a solder assembly step (i.e., a step of providing a purposed member by using a solder). Due to this, the solderability of the Sn plated layer disadvantageously deteriorates.
In the case of a lead wire for a condenser, padding is applied to the portion where the lead wire is welded to a welding target, for example, an aluminum wire. As a result, the plated layer becomes thicker. If the above-mentioned reflow processing is conducted on such a lead wire, the padding of the Sn plated layer becomes disadvantageously nonuniform in thickness.
Meanwhile, if a plated layer is made of a Sn alloy, no whiskers grow up unlike the Sn plated layer. The typical Sn alloy mentioned above is a solder (Sn-Pb alloy), and it has been widely used.
However, it is known that Pb contained in the solder has an adverse effect on human bodies. For that reason, in spite of its excellent properties, use of the solder is shunned. Recently, therefore, Sn alloys which do not contain Pb, that is, a Sn-Ag alloy, a Sn-Bi alloy, a Sn-In alloy and a Sn-Zn alloy have taken the place of the Pb-containing Sn alloy.
However, the lead material having a plated layer formed of any of those Sn alloys, has the following problems.
First, melting points of those alloys are relatively low. Due to this, Cu which is a constituent material for a conductive substrate thermally diffuses to the surface of the Sn alloy plated layer by heat applied during the solder assembly step, and resulting in the solderability of the Sn alloy plated layer to disadvantageously deteriorate.
Furthermore, when a lead wire is welded to, for example, an aluminum wire, the temperature of the welded portion is as close as 2000.degree. C. in a moment and therefore elements such as Zn, Bi and In within the Sn alloy plated layer evaporate in a moment in the vicinity of the welded portion. As a result, blow holes are generated in the welded portion and the welding strength is lowered. Cu diffuses thermally from the conductive substrate to the welded portion and a Cu-Sn compound layer is formed on the surface of the lead material. This might cause a change in the color of the surface and deterioration of solderability.
Among those alloys exemplified as Sn alloys which do not contain Pb, the Sn-Ag alloy and the Sn-In alloy are expensive besides the above-stated disadvantages. The Sn-Bi alloy has low heat resistance to thereby cause the thermal diffusion of Cu in the conductive substrate and has a low bending property to thereby cause cracks to easily occur on the plated layer. The Sn-Bi alloy also has a disadvantage in that the junction strength of the junction part formed after soldering deteriorates as time passes. Moreover, the Sn-Zn alloy has low heat resistance.